Specifically, in wireless domestic networks, it is well known to the person skilled in the art, that on account of multiple reflections suffered by the signal before reaching the receiver, the polarization of the wave emitted is not conserved. Therefore, the antennas do not need to exhibit high purity of polarization. However, the frequency bandwidth demanded may be large. Specifically, for wireless domestic networks at 5 GHz, two disjoint frequency bands have been allocated in Europe according to the BRAN/HIPERLAN2 standard and in the United States, according to the IEEE-802.11A standard. Therefore, to completely cover these frequency bands, the antenna has to operate over a bandwidth of at least 575 MHz for Europe and at least 675 MHz for the United States. Consequently, the frequency band must be respectively around 11% and around 12.3% of the operating frequency.
Furthermore, if one wishes to produce equipment at low cost and in large number using these antennas, additional margins are required in order to take account of the influence of the variations on the parameters of the substrate and of the manufacturing tolerances on the centre frequency of the antenna. Therefore, the relative bandwidths sought are of the order of 15 to 20%.
Moreover, in order to make low-cost and compact antennas, it is known to use antennas of the printed antenna type. However, printed antennas operate in a narrow frequency band. More particularly, the performance in terms of bandwidth, namely the frequency band for which the reflection coefficient S11 at the point of excitation of the antenna is less than −10 dB, are fixed mainly by the parameters of the substrate used such as the relative permittivity, the thickness or the like and the choice of radiating element, which may be a patch, a slot or the like.
However, among printed antennas, it is known that the antennas of the slot type make it possible to obtain simple antenna structures at low cost exhibiting relatively larger bandwidths than the other printed structures.
It is also known that the antennas of the slot type, more particularly antennas constituted by an annular or polygonal slot, can radiate according to a circular polarization. In this case, the circular polarization can be obtained in two ways:
1/ by excitation at two points of two waves with orthogonal linear polarization of like amplitude and exhibiting a phase shift of 90°, as described for example in patent WO94/19842 in the name of THOMSON multimedia;
2/ by excitation at one point, the generation of the circular polarization being obtained by the introduction of a perturbation such as a notch or a protuberance in a plane situated at 45° from the point of excitation.
An antenna of this type is represented in FIGS. 1a and 1b which relate respectively to a plan view from above and to a sectional view of an antenna of annular slot type, fed by microstrip line, furnished with notches to obtain a circular polarization.
More precisely, the antenna is formed by a substrate 1 on one face of which has been deposited a metallic layer 3 in which a radiating element of the annular slot type 2 has been made. This annular slot is fed via a feed line 3 made by metallic deposition on the other face of the substrate 1. This feed line feeds the radiating element 2 by electromagnetic coupling at the point A between the line 3 and the slot 2. The dimension of the line between the point A and the end of the line is around λm/4 where λm is the guided wavelength for the line.
As represented in FIG. 1a, the slot 2 exhibits two diametrically opposed notches 4 lying in a plane situated substantially at 45° from the point of excitation A. Thus, this perturbation makes it possible to separate, in the frequency domain, the two initially degenerate orthogonal modes.
If the two methods described above, making it possible to obtain circular polarization, are compared it is appreciated that, when the circular polarization is obtained by excitation at two points, one obtains better quality of circular polarization over a wider frequency band than when the circular polarization is generated by perturbations in the annular slot.
The method using excitation at two points makes it possible to obtain a good ellipticity ratio or ARBW (standing for Axial Ratio Bandwidth) with a widened adaptation band.